The present invention relates to a semiconductor memory device using a ferroelectric thin film, which is most suitable for a ferroelectric nonvolatile memory or high density DRAM, and to a method of manufacturing the semiconductor memory device.
(1) A conventional ferroelectric thin film capacitor has, as described in “Ferroelectric Thin Film Memory” (published by Science Forum, 1995), page 227, a stacked structure of Pt upper electrode/ferroelectric layer (PZT)/Pt lower electrode.
(2) Based on a surface observation photograph by a scanning electron microscope for a PZT ferroelectric thin film crystallized on a lower electrode, described in Integrated Ferroelectrics, 1995, Vol. 10, pp. 145–154, an average crystal grain size is about 180 nm and a relative standard deviation of crystal grain sizes is about 15%.
(3) In a method of forming a thin film described in Japanese Patent Laid-open No. Hei 7-142600, a compound of BaTiO3 is formed on a Pt thin film, whereby orientation of a ferroelectric thin film is controlled by allowing crystal orientation of the ferroelectric thin film to follow that of the Pt thin film, to thereby ensure remanent polarization.
(4) In an oriented ferroelectric thin film described in Japanese Patent Laid-open No. Hei 6-151601, an epitaxial or oriented buffer layer having a two-layer structure on a semiconductor single crystal substrate and an epitaxial or oriented perovskite ABO3 type ferroelectric substance is formed thereon, to obtain a highly oriented ferroelectric thin film.
In the above references, description is made of the nonvolatile memories using a ferroelectric substance as a capacitor. The problems to be examined, however, are also present in DRAMs using a ferroelectric substance as a capacitor.
(5) For example, as described in “Ferroelectric Thin Film Integration Technology” (published by Science Forum, 1992), pages 13–16, for a 256 Mb DRAM or the like, an attempt has been made to use a crystal thin film made from a high dielectric constant material such as BaSrTiO3 or the like for a capacitor.
In the above-described references (1) and (2), it is difficult to control the crystal grain size and orientation of the ferroelectric thin film. When such a ferroelectric thin film is patterned to form a memory capacitor, a variation in characteristics between memory cells becomes large because of a large variation in crystal grain size, a larger variation in crystal orientation, and a larger surface roughness of each of the ferroelectric thin film and an electrode. This makes it difficult for all of the memory cells to equally obtain sufficient characteristics, giving rise to a problem in exerting adverse effect on the stability in manufacturing yield.
In the above-described reference (3), a variation in orientation is reduced; however, since a variation in grain size of crystal grains in a memory cell is large, a leakage current occurs, an effective voltage between capacitors is reduced because of concentration of an electric field at a grain boundary portion present in the thin film in the film thickness direction, or remanent polarization becomes uneven, which results in degradation of the performance of the memory cell.